1. Field of the Invention
This invention is in the field of nondestructive testing by use of a resonant ultrasound spectroscopy (RUS). RUS is to be distinguished from ultrasonic testing in that it subjects an object to ultrasonic vibrations and which detects ultrasonic vibrations emitted at a natural frequency of the object.
The term RUS (resonant ultrasound spectroscopy) is defined as: an application of a continuous sweep of frequencies as shown in U.S. Pat. No. 4,976,148 or in steps as shown in U.S. Pat. No. 5,408,880, and the measurement is observation of the response of the object to the frequency which is applied.
2. Prior Art
The art of resonant ultrasound spectroscopy has been developed as previously disclosed in U.S. Pat. Nos. 4,976,148; 5,408,880: 5,355,731; 5,062,296; and 5,425,272. Each of these patents is incorporated herein by reference.
In addition, the Assignee of this application has filed a U.S. patent application Ser. No. 08/075,159 filed Jun. 10, 1993 entitled "Method for Resonant Measurement", now U.S. Pat. No. 5,495,763 and U.S. patent application Ser. No. 08/409,218 filed Mar. 23, 1995, now U.S. Pat. No. 5,631,423 entitled "Method for Resonant Measurement" which cover further improvements in the RUS technology. These pending applications are also incorporated herein by reference.
Resonant Inspection using Resonant Ultrasound Spectroscopy (RUS) uses higher order modes (high frequencies) to detect small defects. RI does this by transmitting CW energy to the object. The frequency is swept across the frequency range of interest. The part vibrates when the drive matches the frequency of one of its characteristic modes. Each mode is measured independently and RI uses sensitive receivers, to accurately measure hundreds of modes. As a result, RI can measure very small defects (or small changes in dimensions).
There are major differences among other prior art techniques in how the energy is used to detect defects. Prior art systems include ultrasonic time of flight, ultrasonic imaging, and use of impulse vibrations. Each of these are discussed below.
Ultrasonic Time of Flight is essentially an acoustical radar (more properly, a sonar). A pulse of acoustical energy is transmitted into the object being tested. The pulse travels through the object until it reaches a discontinuity, such as the other side of the object or a flaw within the object. The discontinuity reflects the pulse and the return (echo) is sensed. The time between the transmission of the pulse and its return, is proportional to the distance to the discontinuity. Application of this technique to a wall as the transmitter/receiver moves along the wall, can be effective for sensing the thickness of the wall, and can present a profile of the wall thickness. The technique is referred to by those in the art as ultrasonic because the pulse widths must be short compared to the propagation time, which corresponds to frequencies beyond human hearing (&gt;20 kHz).
Ultrasonic Imaging is where the receiver is placed on the other side of the object from the transmitter, that is opposite the transmitter. If the setup is such that the reflections off the surfaces can be ignored, and the transmitter and receiver are scanned in step, across the surface, then the signal obtained by receiver is an outline (image) of the discontinuity. Again practical considerations require that ultrasonic frequencies be used.
Impulse Vibrations applies a pulse of acoustical energy, usually by striking a sharp blow to an object. This approximates a mathematical impulse function, so the response is the transfer function of the object. In response, the object vibrates in the form of a damped sinusoid which contains all of the frequencies in the transfer function, that is all of the object's vibrational modes. If a defect is present the modes should be effected. It is difficult to obtain any useful information about defects directly from the damped sinusoid. Usually, this time domain wave form is transformed to the frequency domain using a Fourier Transform. However, the amplitude of the sinusoid is dominated by the amplitude of the first resonant mode and the process is inherently noisy, so only the first few modes can be accurately computed. This is sometimes enough to detect large defects, but not small defects.